5.4.3 Possible Exposure Misclassification Radiation doses received by residents on the otheratolls may have been as high or higher than the residents of Utirik. The residents of other atolls were not evacuated as were those on Rongelap and Utirik and may have had continued exposure. 5.4.4 Pooled Exposure Data The use of pooled urine samples and population dose estimates resulted in the inability to assign dose estimates to individual Marshallese Islanders. 5.4.5 Uncertainties in Instrument Calibration Instruments used for measuring exposure were uncalibrated in some instances. Variations and errors in the collection and analysis of urine samples used in the radiation dose estimates also were reported. 5.4.6 Possible Case Ascertainment Bias e Diagnostic differences may exist between the exposed and unexposed control populations because medical services have been provided by different systems. The unexposed control group has been referred to the Marshallese health care system if further work-up was indicated. Further referrals are made on the basis of priorities established by a medical committee in Majuro. The exposed group has been referred to tertiary care centers in the U.S. by a medical screening team. e Regular participation in a medical screening program may reduce all-cause death rates among Marshall Islanders. If so, the result may be underestimation of the effects of radiation. e Vital record information was obtained from informal sources. Because medical visits were periodic, interim cause of death information was obtained largely from 34 VO ZuI